I don't know if this is related to your point exactly, but people outside Captains were requesting better "middle infantry" for each faction, so TW changed/buffed the Vlandian Shock Infantry, yet this wasn't what anyone asked for specifically, and I think it ended up as a failed attempt at a band-aid fix.
I can imagine what people wanted as decent "middle infantry" was basically what you can afford for 1000 gold as infantry in Warband (i.e. 1h sword, decent shield, polearm, middle armour, middle helmet). Otherwise, maybe something similar to the Wildling would have sufficed? Basically, a troop that isn't as expensive as heavy infantry, isn't as buffed as heavy infantry, yet is much more reasonable than the recruit infantry.
Yet, it seems they can't make an entirely new troop for any given faction, with the rigid class system (another reason it's terrible), so they tried turning the Shock Infantry into a decent "middle infantry" for Vlandia, which I don't think worked out well at all.
The real issue is just how pathetic the recruit infantry is. It's terrible as a minimum loadout. It doesn't even work as a joke loadout (similar to playing naked), because it's not a customisation by the player, it's a prebuilt troop made by TW, and it's forced on the player like a slap in the face sometimes, which is terrible for new players.
It would definitely serve Siege/TDM better if recruit troops were improved into something more reasonable, while Shock Infantry can be nerfed (along with the removal of crushthrough).
In any case, I agree with you that the modes should be split. It's been brought up plenty of times since beta well over a year ago, and people responded very negatively to the class/perk system since early 2018, due to the horrible impact it would have on MP overall.